The following article explains how to convert your grid or graticule to graphic and 'send to back' so it draws underneath the layers in your map:
Place a grid or graticule below the data layers in the map composition -
http://support.esri.com/fr/knowledgebase/techarticles/detail/17370
However, if you have a raster in your table of contents, the grid/graticule disappears completely underneath the raster. It would be great to have a way to send the grid/graticule back behind select layers of your choosing (so, you can specify to send the grid/graticule behind labels only).
